1. Field of the Invention
This invention relates to a crystal growth method by which a good quality single-crystal film is heteroepitaxially grown on a single-crystal substrate. More particularly, it relates to a crystal growth method by which the direction of the plane of the single-crystal film is inclined at a certain angle from the direction of the plane of the single-crystal substrate or the single-crystal film is grown on a selected plane direction of the single-crystal substrate.
2. Description of the Prior Art
Silicon carbide (SiC) is a semiconductor material, which has a wide forbidden energy gap of 2.2 to 3.3 electronvolts (eV) and is thermally, chemically and mechanically stable and also has a great resistance to radiation damage. Both p-type and n-type silicon carbides have good stability, which is rare for widegap semiconductors, making them useful as a semiconductor material for electronic devices operable at high temperatures or with great electric power, for highly reliable semiconductor devices, for radiation-resistant devices, etc., and usable in an environment where difficulties are encountered with devices made of conventional semiconductor materials, thereby greatly enlarging the range of application for semiconductor devices.
Silicon carbide is also useful as a semiconductor material for optoelectronic devices utilizing visible light of short wavelengths and near-ultraviolet light with the use of its wide energy gap. Whereas other wide-gap semiconductor materials such as semiconductors made of II-VI groups, III-V groups, etc., generally contain heavy metals as a main component therein and thus essential problems of pollution and availability of raw materials arise, silicon carbide is free of these problems. Moreover, silicon carbide has many variant structures (i.e., polytype structures).
Despite these many advantages and capabilities, silicon carbide has not yet been placed in actual use because the technique for growing silicon carbide crystals with good reproducibility which is required for commercially producing high quality silicon carbide substrates having a large surface area has yet to be developed.
Conventional processes for preparing single-crystal substrates of silicon carbide on a laboratory scale include the so-called sublimation method (i.e., the Lely method) wherein silicon carbide powder is sublimed in a graphite crucible at 2,200.degree. C. to 2,600.degree. C. and recrystallized to obtain a silicon carbide substrate, the so-called solution method wherein silicon (Si) or a mixture of silicon with impurities such as iron, cobalt, platinum or the like is melted in a graphite crucible to obtain a silicon carbide substrate, and the Acheson method which is generally used for commercially producing abrasives and by which silicon carbide substrates are obtained incidentally.
Although a large number of crystals can be obtained by either the sublimation method or the solution method, it is difficult to prepare large single-crystal substrates of silicon carbide since many crystal nuclei occur at the initial stage of crystal growth. Silicon carbide substrates incidentally obtained by the Acheson method are so inferior in purity and crystallinity that they cannot be used as semiconductor materials. Even though large single-crystal substrates are obtained, they are only incidental and therefore, insignificant to commercial production of silicon carbide substrates. Thus, according to these conventional processes for the production of single-crystal substrates of silicon carbide, it is difficult to control the size, shape and quality of single-crystal substrates of silicon carbide on an industrial scale.
On the other hand, in recent years, with advances in semiconductor technologies, it has been possible to obtain thin single-crystal films of 3C type silicon carbide (which has a cubic crystal structure and an energy gap of 2.2 eV) on silicon substrates, which have been available as large single-crystal substrates, by a heteroepitaxial technique using chemical vapor deposition. Chemical vapor deposition is a crystal growth technique that is excellent in mass-production on an industrial scale and attains the reproducible growth of high quality silicon carbide single-crystal films with a large surface area on silicon substrates. For the epitaxial growth of a thin single-crystal film of 3C type silicon carbide on a silicon substrate, SiH.sub.4, SiCl.sub.4, SiH2Cl.sub.2, (CH.sub.3)3SiCl, (CH.sub.3).sub.2 SiCl.sub.2 or the like are used as a silicon material; Cl.sub.4, CH.sub.4, C.sub.3 H.sub.8, C.sub.2 H.sub.6 or the like are used as a carbon material; hydrogen, argon or the like are used as a carrier gas; and the temperature of the silicon substrate is set to be 1,200.degree. C. to 1,400.degree. C. However, since constituting the silicon substrate is a different material from silicon carbide constituting the single-crystal film, silicon is poor in wettability to silicon carbide. Moreover, the lattice constant of silicon is different from that of silicon carbide by as much as 20%. Accordingly, strain and/or stresses arise in the growth film, which causes warpage and/or cracks and, furthermore, silicon carbide grown on the silicon substrate cannot be single crystal films with a layered structure, but are polytype crystals with a dendrite structure. Even though a thin single-crystal film of silicon carbide is obtained on the silicon substrate, the quality of the crystal deteriorates with an increase in the thickness of the crystal film, resulting in a polytype crystal.
With a GaAlAs single crystal formed on a GaAs substrate and an InGaAs single crystal formed on an InP substrate, which have been put into practical use as a semiconductor material, the difference in lattice constant between the substrate and the growth film is as small as 1% or less, which causes none of the problems mentioned above. However, other combinations of substrates and growth films bring about the abovementioned problems because of different lattice constants therebetween, which causes difficulties in putting them into practical use. FIG. 3 shows the crystal direction of a conventional heteroepitaxial growth, indicating that when a single-crystal growth film B with the lattice constant b is formed on a single-crystal substrate A with the lattice constant a (a&lt;b) by a heteroepitaxial growth technique, the crystal direction of the substrate A is the same as that of the epitaxial growth film B and thus there is a difference in lattice constant at the interface C between the substrate A and the film B, resulting in lattice distortion, which causes crystal defects.
An improved chemical vapor deposition method has been proposed by Appl. Phys. Lett., 42(5), Mar. 1, 1983 p460-p462, wherein the surface of a Si single-crystal substrate is heated and carbonized within a hydrocarbon gas atmosphere, resulting in a SiC thin film on the said surface, and then a silicon material gas and a carbon material gas are supplied thereto to form a SiC single-crystal. The plane directions of the said Si substrate used as a crystal growth substrate are (100) and (111) planes. When SiC is grown on the (100) plane of the Si substrate, microscopic defects such as an antiphase boundary, etc., occur at the silicon-silicon carbide interface. When SiC is grown on the (111) plane of the Si substrate, warpage and/or cracks occur in the resulting SiC growth film. The use of an Si substrate with regard to the (100) and (111) planes as an underlying substrate for crystal growth does not provide the excellent semiconductor material that is essential to obtain semiconductor devices with improved device characteristics.